An early circa 1960 installation by Pasveer for waste water, i.e., primarily sewage, purification by the activated biological sludge included a closed circuit or ditch with a motor-driven horizontally-rotated brush used for adding needed oxygen (air) to the sewage and moving the sewage in circulation. In U.S. Pat. No. 3,510,110, an orbital system employing an elongated tank with central partition was disclosed which employed a vertically-rotated surface aerator located at the end(s) of the partition wall for both aerating the sewage and circulating the sewage around the channels formed by the partition wall and the sides of the tank.
This latter system has had great commercial success with over 500 plants in operation world-wide (ranging from less than 1 MGD capacity to one of over 10 million population equivalent) with over 250 plants in operation or in various construction phases since 1976 in the United States up to the present time. Sold under the trademark Carrousel.RTM., the high popularity of the system is due primarily to its cost-effectiveness, simplicity of design, ease of operation and maintenance, and excellent effluent quality. It can treat raw domestic water to EPA advanced secondary standards without primary clarifiers or effluent filters. With extended aeration, it produces a highly stable water sludge requiring little or no further processing prior to disposal. Carrousel.RTM. systems can be designed to have a power turn-down of 50-75%. Aerator drive horsepower can be varied from 100% of installed capacity to as little as 25% without loss of mixing and continuing sufficient mixed liquor channel velocity. This power down flexibility provides an ability to closely match oxygen input to the mixed liquor to oxygen demand of the microbes acting to degrade the sewage, without loss of mixing and movement.
In U.S. Pat. No. 4,869,818 a radial flow high efficiency submerged impeller is added to the same shaft as drives the surface aerator so that mixed liquor in the lower portion of the orbital channels is pumped in the same direction as that mixed liquor pumped by the surface aerator. This effectively alleviates certain depth restrictions in orbital tanks resulting in deeper channels requiring inter alia less concrete and less land space.
Various prior art wastewater treatment systems and methods employ a first reaction vessel where a mixture of wastewater and micro-organisms which "feed" on the organic material in the wastewater in the presence of air or oxygen from an aerator, are aerobically treated. A portion of the mixed liquor is then processed in a second clarifier vessel under anoxic conditions. The first reaction vessel may be of the orbital-type such as a Carrousel.RTM. system.
U.S. Pat. Nos. 4,303,516 (Stensel et al.) and 4,818,391 show the incorporation of a clarifier within the orbital tank, the latter showing the formation of an aerobic zone along one part of the orbital path, an upstream aerator and an anoxic zone downstream of the aerobic zone (FIG. 16). In the '391 patent FIGS. 1-12 show a large rectangular tank including a rectangular reaction vessel and a rectangular clarifier in communication with one another. As the oxygen supplied by the aeration device is consumed in the aerobic zone, the reaction become anaerobic, i.e., anoxic in the downstream anoxic zone. The micro-organisms then utilize the oxygen bound in the nitrates (NO.sub.2 and NO.sub.3 compounds) formed in the aerobic zone, to reduce the nitrates to nitrogen in the anoxic zone for release to the atmosphere as a gas. The mixed liquor then enters the downstream clarifier. Clarified liquor is removed while the settling biological solids are returned directly (recycled) to the main flow of mixed liquor travelling through and around the orbital path.
Devices shown in U.S. Pat. Nos. 4,303,516, 4,446,018, 4,818,391 and 4,383,922 involve interchannel clarifiers and except as noted above do not suggest or discuss a separate anoxic zone.
U.S Pat. No. 4,543,185 discusses the use of a barrier downstream of an aerator in an orbital ditch for preventing stratification of aerated liquor in the ditch top and unaerated liquor flow near the ditch bottom. The '185 patent also discloses detoxification employing alternating aerobic-anoxic sequences of stages without intermediate clarification, to effect nitrogen removal, i.e., denitrification. U.S. Pat. No. 4,159,243 (Okey) also discloses the use of an anoxic zone in an orbital channel, downstream from the aerobic zone, for de-nitrification. Netherlands Patent No. 8301-500-A discloses an orbital channel by-pass which provides alternate nitrification and denitrification.